Aw, if you're using an AVR8 to connect a sensor over USB there are better ways than using a serial over USB.
It would be better to use an AVR8 with a USB interface, like a ATMega32u4 (Leonardo in Arduino-speak, I think).
Using serial over USB gets into problems of framing/waiting for the 0x0a (newline or whatever).
Use the USB HID sensor protocol.
Ok, you don't have to get too serious about the descriptors, just make send things in regular HID reports.
Neat, clean, you never get half a report, like with serial over USB.
The one exception to all this is if you want/had to use a serial optoisolator, in which case serial out of the AVR8 to isolator to FTDI would work.
You could also use a USB optoisolator too.